RF filters and duplexers have been essential components of communication systems. High selectivity, low insertion loss, compact size, ability to handle large signals (power handling), high linearity, manufacturability, and low cost may be some of the important desired features for RF filters and duplexers.
The requirements for RF filters and duplexers have become more stringent in light of new communication standards where information channels and frequency bands are closer to each other, new communication devices such as smartphones where the footprint and cost of all components must be very small as more components are needed in support of multiple standards and applications, and co-existent communication systems where multiple communication transmitters and receivers work simultaneously.
Linearity, noise, and power handling requirements typically lead to utilization of passive RF filters and duplexers in many applications. The performance of passive RF filters and duplexers may be limited by the quality factor (Q) of the components that are used in their realization. The filter selectivity as well as passband requirement may lead to a filter topology and filter order. For a given RF filter or duplexer topology and order, insertion loss may reduce with the increase of component Q.
Various technologies can be used to realize passive RF filters and duplexers. For instance, capacitors, inductors, or transmission lines can be used to realize passive RF filters and duplexers. Electromagnetic resonators, including transmission line and dielectric waveguide resonators, can also be used to realize passive filters and duplexers. The quality factor of such components is typically proportional to their overall physical size. As such, it has been difficult to realize compact low-loss selective passive RF filters and duplexers using electromagnetic components and resonators.
Piezoelectric material can be used to realize compact high-Q resonators. Crystal resonators have been widely used to generate spectrally-pure oscillators. Surface acoustic wave (SAW) resonators have been widely used to realize compact low-loss selective RF filters and duplexers as well as oscillators. More recently, bulk acoustic wave (BAW) resonators have been used to construct high-performance RF filters and duplexers as well as oscillators.
Ceramic resonators and micro electro mechanical system (MEMS) resonators with high quality factor have also been used in frequency generation as well as filtering applications.
RF SAW filters and duplexers have been used widely in wireless communications such as cellular phones, wireless local area network (WLAN) transceivers, global positioning system (GPS) receivers, cordless phones, and so forth. RF SAW filters have been used as band-select filters, image-reject filters, intermediate frequency (IF) filters, transmitter noise or spur reduction filters, and so forth. A typical smartphone may have several SAW resonators, SAW filters, and SAW duplexers to support various communication systems and standards.
Over the past decade, significant research and development on BAW technology has resulted in BAW resonators that have lower loss (or higher Q) or are more compact, especially at higher frequencies, compared with SAW resonators. Therefore, RF filters and duplexers that use BAW resonators may have lower insertion loss, or higher selectivity, or smaller form factor compared with those that utilize SAW resonators especially at higher frequencies. Thin film bulk acoustic resonators (FBAR) and solidly mounted resonators (SMR) are a common example of BAW resonators.
Modern wireless communication standards designate many different operational frequency bands to support the increase in the overall wireless capacity and reach. For instance, cellular phone standards may include RF frequency bands that span around 700 MHz to around 4000 MHz. Furthermore, in order to increase the overall wireless capacity, the frequency spacing between adjacent frequency bands or channels within the same application or different applications may be reduced. This may be done, for instance, by reducing the typical guard bands in wireless standard or by placing the transmit and receive frequency bands in a frequency division duplex (FDD) scheme closer to each other. As a result, RF filters and duplexers with higher selectivity may be required. More selective RF filters and duplexers that utilize a given component or technology (SAW, BAW, etc.) may incur more in-band insertion loss. The higher RF filter or duplexer insertion loss may reduce the wireless receiver noise figure and sensitivity, increase the wireless transmitter power consumption or reduce the transmitted power, and/or deteriorate the overall performance of a communication system.
In commercial systems, the choice of technology may depend on the technical performance, such as power consumption as well as economic and business considerations such as cost, size, and time to market. For instance, while one technology may offer a better performance compared with another technology, it may not be adopted for a commercial system that is cost sensitive. In the case of RF filters and duplexers, it may be desirable to use a technology that leads to the lowest-cost and/or most-compact solution, as long as a predetermined performance criterion is met. In other words, a more expensive or larger solution may not be adopted, even if it offers a better performance as compared with an alternative solution that meets an acceptable performance level at a lower cost and/or size. For instance, while RF filters and duplexers that use BAW resonators may offer lower loss compared with RF filters and duplexers that use SAW resonators for a given set of specifications, the higher relative cost of BAW technology, as well as its relatively smaller number of suppliers, may disfavor their usage in certain applications and standards. Other considerations may be the ease of integration with the rest of the components in a communication system. For instance, there may be performance, business, or economic advantages to integrate RF filters and duplexers with low noise amplifiers (LNA), power amplifiers (PA), transmit/receive (T/R) or band-select switches, impedance matching networks, etc. in a compact RF module. A typical modern wireless communication device, such as a smartphone, may have a number of SAW filters and duplexers as well as a number of BAW filter and duplexers. Each SAW or BAW filter or duplexer may be used for a specific communication application, standard, or frequency band.
Architectural solutions that enable realization of highly-selective low-loss duplexers with high-isolation between transmit and receive bands are highly desirable. Specifically, it is highly desirable to use a lower cost or more compact technology within an innovative architecture that satisfies a comparable or better specification compared to what can be achieved using a more expensive or less compact technology. Examples might include replacing BAW duplexers with SAW duplexers using an innovative architecture, or replacing ceramic or cavity duplexers with BAW duplexers using an innovative architecture.
A conventional method to design acoustic resonator based filters and duplexer is to decide upon the number of resonators to be used depending on the required stopband rejection in the case of filters or the required isolation in the case of duplexers. The larger the number of resonators used in filter design, the larger may be the order of the filter and the higher may be the rejection of out-of-passband frequencies (or higher stopband rejection). Similarly, the number of resonators used in the TX and RX filters of the duplexer may determine the total isolation from TX to RX. The larger the order of the TX and RX filters (i.e., the larger the number of resonators used in them), the larger may be the amount of isolation between TX and RX. Due to the limited quality factor of the acoustic resonators, the insertion loss in the filter and duplexer may be directly proportional to the number of the resonators used. In other words, the larger the order of the filter and the TX and RX filter, the larger may be the loss of the filter and duplexer, respectively. It may be possible to break this insertion loss and isolation or stopband rejection tradeoff by incorporating hybrid couplers in the design of filters and duplexers.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with the present disclosure as set forth in the remainder of the present application with reference to the drawings.